It is important to predict whether or not fluid injection into or near a fault will cause slip, since this can result in seismicity of sufficient magnitude to cause damage to surface structures. An important mechanism that is usually neglected when analyzing fluid injection into faults is the leak-off into the surrounding rock mass. The objective of the study is to outline the importance of considering this leak-off mechanism. This is achieved by comparing a two-dimensional fully coupled fluid and mechanical loading extended finite element method (X-FEM) formulation (via development of a standalone code in MATLAB) with the Universal Distinct Element Code (UDEC). The X-FEM code has the capacity to consider leak-off into the surrounding rock mass from the fault, whereas UDEC does not have this capability unless extremely fine blocks are generated in the vicinity of the fault to simulate a fracture network within the fault damage zone. A case study is presented and the results indicate that using the UDEC the normal and shear displacements are comparable to an impermeable X-FEM model. However, there is additional pore pressure produced at the center of the fault in the impermeable X-FEM calculation, compared with the UDEC. When the surrounding rock mass has high permeability this pore pressure reduces in the X-FEM model, producing less slip than the UDEC result. This X-FEM provides an advancement over previous studies that do not consider leak-off from the fault into the surrounding rock mass. The proposed method may assist with providing a more accurate prediction of fault-slip. This slip mechanism is important to understand in the context of induced seismicity and its associated concerns. 1 Introduction Seismicity large enough to cause damage on the surface can be caused from fluid injection into or near a fault, generating slip (Raleigh, Healy & Bredehoeft, 1976; Nicholson & Wesson, 1992; Cornet et al., 1997; Majer et al., 2007; Ellsworth, 2013). The complexity due to the fluid-rock mechanical interaction makes it difficult to predict seismic events from fluid injection. Therefore, to predict this seismicity the fault mechanics due to fluid injection need to be determined. To predict fault movement these coupled fluid and mechanical processes are needed to be used in the analysis. Previous studies have predominately focused on developing numerical methods (Rutqvist et al. 2002) to generate parametric results (Rutqvist et al. 2013, 2015) however, these do not compare different numerical methods. This study introduces a coupled extended finite element method (X-FEM) approach that considers the fluid-rock mechanical interaction, including the leak-off into the rock mass. The method should be sufficiently efficient to be used in industry and uses appropriate rock properties as inputs into the numerical model. This study concentrates on the comparison of this developed code with the Universal Distinct Element Code (UDEC) (Cundall 1980) and provides evidence that the leak-off into the rock mass is an important process governing the fault mechanics due to fluid injection.

The Silurian Longmaxi shale formation is a widely distributed gas reservoir in the Sichuan Basin of southwest China. The thick clay rich shale has relatively high organic content, is thermally mature and is a major target for China's shale gas production. In recent years the shale has become the host to an increasing number of induced earthquakes potentially linked to fluid injection related to hydraulic fracturing. These induced or triggered events are linked to the reactivation of faults. This paper explores the frictional and stability properties of Longmaxi shale to provide insights into the causal mechanisms of induced seismicity and their geological and structural controls. Rock samples from a geological section spanning ~150 m were powdered for friction experiments at recreated in-situ stresses and temperatures. Experimental results show that fault frictional and stability properties are strongly controlled by mineral composition. Frictional strength increases with a reduction in phyllosilicate content and exerts a significant control on seismicity. Two of the ten samples exhibited velocity weakening behavior under in-situ conditions, potentially identifying compositions that contribute to unstable slip. These findings have important implications in understanding the frequency and occurrence of induced earthquakes triggered as a result of hydraulic fracturing. 1. Introduction The rapid growing earthquakes induced by shale gas hydraulic fracturing have aroused wide concerns in recent years (Bao and Eaton, 2016; Ellsworth, 2013; Lei et al., 2017). The large-scale fluid injection can generate high pore pressure in the subsurface and may activate the pre-existing faults (Elsworth et al., 2016). Previous researches demonstrated that the fault sliding behavior is governed by the frictional properties of fault gouge (Kohli and Zoback, 2013; Zhang et al., 2019). Thus, a careful examination of the fault gouge friction properties is important for understanding the underlying mechanism of fault activation and mitigating the seismic hazard during shale gas fracturing. The Silurian Longmaxi shale is a major target for shale exploitation in southwest China. In this work, the Longmaxi shales recovered from the outcrop in Sichuan Basin, southwest China were sieved to simulate the fault gouge. The friction experiments were conducted on these simulated fault gouges at hydrothermal conditions to explore the effect of mineralogy on gouge frictional and stability properties.

ABSTRACT: Seismic monitoring offers the best insight into current rock mass damage process and failure related to mining-induced fault reactivation. Brittle failure can be recorded in real-time as seismic events. This paper explores the relationships between geological structure and mining-induced seismicity through real time monitoring in an area called sanshandao mine, with concentration on the 645 level. Geological features within Sanshandao mine have a reported association with seismic activity, 406 orebody shear zones were identified during field investigations, the most prominent striking SW and steeply dipping NW. Seismicity from 2013.11 - 2014.4 is analysed, Spatial and temporal trends and seismic event parameters show little correlation to shear zone geometry. Instead, seismic event parameters correlate to spatial clusters of events. 1 INTRODUCTION Seismic monitoring offers the best insight into current rock mass damage process and failure related to mining-induced fault reactivation. Brittle failure can be recorded in real-time as seismic events. The degree of damage in a rock mass can have a dramatic effect on the properties of the seismic waves emitted from microseismic sources, notably on wave velocity and attenuation (Feustel 1998). Such changes have been used to describe the rock mass character. Lower velocity and higher attenuation is recorded in a heavily fractured rock mass as compared to a homogeneous and unfractured rock mass (Jiang 2003). Given this, it is expected that the state of damage in the rock mass would also have a noticeable effect on source parameters, most of which are calculated directly or indirectly from the recorded waveforms. Temporal trends in seismic event parameters can approximate loading curves similar to those traced in acoustic emission tests (Coulson & Bawden 2008). Such curves indicate that at the point of fracture initiation, the moment magnitude, seismic moment, seismic energy and apparent stress increase until the point of yield, at which point fractures coalesce and there is a significant drop in parameter values (Coulson & Bawden 2008). Spatial and temporal analysis of microseismic event parameters was conducted to identify trends and assess rock mass properties on level645. Temporal analysis of levels and clustered events on the 645 Level did not reveal any significant trends but did reveal comparable spatial event distributions: dense seismicity extends from the southeastern corner of the excavation to an area southwest of the excavation Figure 1. Events in this area occur sporadically during the half-year time period. It is postulated that in the Sanshandao Deep the micros seismic event distribution as well as the event parameters reflect both local stress conditions and the physical state of the rock mass.